Symmetrical planar diac

ABSTRACT

The invention concerns at diac comprising a highly-doped substrate ( 20 ) of a first type of conductivity, a lightly-doped epitaxial layer ( 22 ) of the second type of conductivity including in the neighbourhood of the substrate ( 20 ) a more highly-doped part ( 21 ), a highly-doped region ( 24 ) of the first type of conductivity on the side of the upper surface of the epitaxial layer, a region ( 23 ) of the second type of conductivity more doped than the epitaxial layer beneath the region ( 24 ) of the first type of conductivity and not overlapping relative thereto, a channel retaining ring ( 25 ) of the second type of conductivity more highly doped than the epitaxial layer, outside the first region, a wall ( 26 ) of the first type of conductivity outside said ring, joining the substrate.

[0001] The present invention aims at novel diac structures especially enabling easing the assembly of such components.

[0002] A conventional diac structure is shown in FIG. 1. The structure is formed from a substrate 1 of a first conductivity type, here type P. Heavily-doped regions of the opposite type, here, type N, respectively 2 and 3, are formed on either side of the substrate. To obtain a sufficiently high breakdown voltage, the so-called mesa technology is used, which consists of etching grooves at the border between two diacs formed in a same wafer. The angle formed by the groove at the location where it cuts the junction between the P and N⁺ regions is an important parameter for determining the breakdown voltage at the component periphery. Another important parameter is the choice of the passivation product 4 formed in the grooves.

[0003] The upper and lower surfaces of the diac are covered with metallizations M1 and M2. Conventionally, a diac is a device of small size, its thickness being smaller than 0.3 mm and its surface being on the order of 0.5 mm×0.5 mm.

[0004] Special packages are thus provided for the diacs, for example, piston systems arranged on either side of a glass tube in which the chip is enclosed.

[0005] To avoid difficulties associated with the mesa technology and with the digging of grooves, it has been attempted to make diacs of planar type, for example, such as that shown in FIG. 2, also formed from a P-type substrate 1. The upper and lower surfaces of the substrate are coated with a masking layer, for example, silicon oxide, respectively 11 and 12, provided with a central opening through which is formed an N⁺-type diffused region, respectively 13 and 14. These planar structures enable obtaining satisfactory planar structures of the junction peripheries, but pose assembly problems. Indeed, it becomes difficult to weld the chip on a metal support wafer since, in case the welding extends laterally beyond its location, a short-circuit is created between one of the N⁺ regions and the P substrate. Metallizations must thus be provided, for example formed of silver balls 15 and 16, located on N⁺ regions 13 and 14, which complicates the assembly and increases its cost.

[0006] Thus, to assemble a diac of planar type such as that of FIG. 2, very specific packages and assembly modes must be provided.

[0007]FIG. 3 illustrates the typical characteristic of a diac. Such a component cannot be assimilated to two head-to-tail zener diodes. Indeed, the existence, when one of the junctions is in avalanche, of another forward junction which injects into the substrate, causes a breakover type effect. Thus, the diac breaks down when the voltage thereacross reaches a value VBO. The voltage then drops to an intermediary voltage Vf as long as the current is a given range of values. The voltage across the diac rises back if the current comes out of this range. In the example shown in FIG. 3, the value of voltage VBO is 32 volts, the value of voltage Vf is 13 volts, the current at the breakover time is on the order of 0.3 μA (that is, the diac has very slight leaks), and the current corresponding to voltage Vf is a range on the order of from 10 to 100 milliamperes.

[0008] A diac, such as those shown in FIGS. 1 and 2, has a symmetrical characteristic, as shown in FIG. 3. The value of voltage VBO essentially depends on the doping levels of the junctions between the N⁺ regions and the P substrate. The value of forward voltage Vf essentially depends on the doping level and on the thickness of substrate 1, which can be considered as the floating base of a transistor having its emitter and collector corresponding to the N⁺ regions. This base must be such that the carriers injected by the forward junction can cross it. The carrier lifetime must thus be long in the base if its width is large, that is, it must be lightly doped. If the base dimension becomes small, the carrier lifetime in this base must be reduced, for example by a metallic diffusion. These various compromises determine above-mentioned voltage Vf.

[0009] Generally, semiconductor manufacturers are asked to provide diacs having well established values of VBO and Vf. For example, a diac having a voltage VBO of 32 V±12%, with a voltage drop from VBO to Vf of a minimum 10 V, and with a dissymmetry smaller than a few percents of the considered values, will be desired.

[0010] An object of the present invention is to form such a diac which is easy to manufacture, that is, which is of planar type and not of mesa type, and which is easily assembled on a connection grid including a base on which a surface of the component is welded.

[0011] To achieve this object, the present invention provides a diac including a substrate of a first conductivity type with a high doping level, a lightly-doped epitaxial layer of the second conductivity type including in the vicinity of the substrate a more heavily-doped portion, a heavily-doped region of the first conductivity type on the upper surface side of the epitaxial layer, a region of the second conductivity type more heavily doped than the epitaxial layer under the region of the first conductivity type and that does not extend beyond said layer, a channel stop ring of the second conductivity type more heavily doped than the epitaxial layer, outside of the first region, a wall of the first conductivity type outside of said ring, joining the substrate.

[0012] According to an embodiment of the present invention, the first conductivity type is type N.

[0013] The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which:

[0014]FIG. 1 shows a mesa-type diac structure according to prior art;

[0015]FIG. 2 shows a planar-type diac structure according to prior art;

[0016]FIG. 3 shows the current/voltage characteristic of a diac;

[0017]FIG. 4 is a simplified cross-section view of a diac structure according to the present invention; and

[0018]FIG. 5 shows a diffusion profile of an example of forming of a diac according to the present invention.

[0019] As shown in FIG. 4, the present invention provides forming a diac on a structure including a heavily-doped substrate 20 of a first conductivity type, which will be considered hereafter as being type N. A P-type epitaxial layer including a heavily-doped buried layer 21 followed by a lightly-doped layer 22 is formed on this substrate. A region 23 more heavily doped than region 22 is formed in epitaxial layer 22, through a first mask. An N-type region 24 extending on all sides beyond region 23 and more heavily doped than region 23 is formed above region 23, through a second mask. Thus, a portion of P-type doped region 23 remains under N-type region 24. At the periphery of region 24, and separately therefrom, a P-type ring 25 is formed, which has a channel stop function. The component periphery is occupied by a heavily-doped N-type wall 26, which crosses epitaxial layer 22 and joins substrate 20. Wall 26 is external to ring 25 and is separate therefrom. This wall 26 is formed immediately after epitaxial layer 22.

[0020] A metallization M1 is formed on the upper surface of N⁺ region 24 and a metallization M2 is formed on the lower surface of N⁺ substrate 20. Thus, a diac is obtained between metallizations M1 and M2. Channel stop region 25 has the function of avoiding for leakage currents to flow in a region located under the upper surface of epitaxial layer 22 from metallization M1 to metallization M2 via wall 26 and substrate 20. Wall 26 has the function of avoiding for the junction between substrate 20 and epitaxial layer 22 to emerge on the outside of the component.

[0021] The first junction of the diac corresponds to the junction between N⁺ region 24 and P region 23, and the second junction of the diac corresponds to the junction between epitaxial layer portion 21 and substrate 20. The structure of this diac enables metallization M2 to be welded on a metal base possibly belonging to a connection grid. Indeed, even if weldings extend laterally, and up on the diac walls, they cannot create short-circuits since the lateral walls are uniformly of type N⁺ like the layer in contact with metallization M2.

[0022] The positive avalanche voltage (taking metallization M2 as a reference) depends on the doping gradient between P region 23 and N⁺ region 24. The negative breakdown voltage (still taking metallization M2 as a reference) depends on the doping gradient between portion 21 of the epitaxial layer and N⁺ substrate 20. The breakdown voltages can be made equal by properly choosing the doping profile of the various layers.

[0023]FIG. 5 illustrates an example of a doping profile of a diac according to the present invention having a characteristic such as shown in FIG. 3.

[0024] In FIG. 5, the abscissas correspond to vertical distances in micrometers, value “0” corresponding to the upper surface of the N⁺ substrate on which the P-type epitaxial layer is developed. The profile of this epitaxial layer corresponds to what is indicated by reference 41. The doping level of this layer rises at the level of substrate 20 due to the presence of buried layer 21 and at the level of region 24 due to the presence of diffused region 23. During the epitaxy, an N⁺-type region of substrate 20 diffuses according to the curve designated by reference 42. Curve 44 corresponds to N⁺-type diffusion 24 formed from the upper surface.

[0025] The references corresponding to the various regions have been indicated under the abscissas. Thus, N⁺-type layer 24 extends at approximately 5 μm under the upper surface of the epitaxial layer, and P-type region 23 and P-type layers 22 and 21 extend down to substantially 14 μm from the epitaxial layer surface. The doping level of P-type layer 21 at the junction with N⁺-type substrate 20 and the doping level of P-type layer 23 at the interface with region 24 both are on the order of 5.10¹⁶ atoms/cm³.

[0026] Of course, the doping profile of FIG. 5 is an example only and the various doping materials may be optimized according to the desired diac characteristics. 

1. A diac including: a substrate (20) of a first conductivity type with a high doping level, a lightly-doped epitaxial layer (22) of the second conductivity type including in the vicinity of the substrate (20) a more heavily-doped portion (21), a heavily-doped region (24) of the first conductivity type on the upper surface side of the epitaxial layer, a region (23) of the second conductivity type more heavily doped than the epitaxial layer under the region (24) of the first conductivity type and that does not extend beyond said region, a channel stop ring (25) of the second conductivity type more heavily doped than the epitaxial layer, outside of the first region, a wall (26) of the first conductivity type outside of said ring, joining the substrate.
 2. The diac of claim 1, characterized in that the first conductivity type is type N. 